Filter mock_ai-related metrics events from telemetry

[svarlamov]

Summary

Closes #976. Prevents mock_ai test preset events from reaching telemetry by filtering in observability::log_metrics() — the single lowest-level funnel for all metric events.

A pub(crate) is_mock_ai() helper in src/metrics/mod.rs inspects the serialized MetricEvent and checks two positions:

1. Attr position 20 (TOOL) — catches AgentUsage / Checkpoint / InstallHooks events where attrs.tool = "mock_ai"
2. Value position 3 (TOOL_MODEL_PAIRS) — catches Committed events whose tool_model_breakdown keys start with "mock_ai" (e.g. "mock_ai::unknown")

A MOCK_AI_TOOL constant in src/metrics/mod.rs centralizes the string. log_metrics() always runs the filter, then delegates to submit_metric_events() — a no-op in test/test-support builds, real submission otherwise. This means is_mock_ai() is always compiled and exercised by tests with zero cfg annotations on the detection logic itself.

Review & Testing Checklist for Human

• starts_with(MOCK_AI_TOOL) matching on tool_model_pairs — verify no real tool name could start with "mock_ai" and get falsely filtered.
• Position constants — the helper uses attr_pos::TOOL (20) and committed_pos::TOOL_MODEL_PAIRS (3). Confirm these match the current sparse array encoding.
• Integration path not tested in CI — submit_metric_events is a no-op in test builds, so the filter→submit→telemetry path is never exercised end-to-end in CI. Recommended manual test: run git-ai with the mock_ai preset, make a commit, and verify no events appear in the metrics DB or telemetry upload logs.

Notes

• Two files touched: src/metrics/mod.rs (MOCK_AI_TOOL constant + is_mock_ai() helper + 5 unit tests) and src/observability/mod.rs (filter call in log_metrics() + submit_metric_events cfg split).
• All existing + new tests pass; lint and format checks clean.

Link to Devin session: https://app.devin.ai/sessions/9e5cb55142ff40b387b12c28ddf113cb
Requested by: @svarlamov

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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